This invention relates to magnetic disks for use as memory storage devices, and more specifically to magnetic disks whose final protective and lubricative layer is carbon.
On a magnetic disk there may be millions of magnetized regions, each region representing one bit of information. To write to and read from the disk, a magnetic transducer, or head, is supported in close proximity to the disk surface by a cushion of air created by the swiftly rotating surface of the disk. Rotational speeds are typically 60 revolutions per second, and the flying height may be a quarter of a micrometer or even less.
When the disk spindle is powered off or powered on there is insufficient rotational speed to generate the supporting cushion of air for the head to fly above the surface, and the head, with a load typically on the order of a few grams, drags on the disk surface. This dragging can cause scratching or wearing of the disk's thin outer protective/lubricating layer (300-800 .ANG.) and may eventually damage regions of the magnetic layer causing erroneous data to be read from the disk. This reading of erroneous data is referred to hereinafter as "disk wear".
Similarly, when the disk is started or stopped, the static friction and stiction between the head and the disk also wears the outer protective layer, eventually wearing through portions of the protective layer and damaging the underlying magnetic layer. In addition, with respect to dynamic friction, once wearing of the outer layer begins, wearing is accelerated due to increased friction created by the increased roughness of the surface. Wear particles also intersperse themselves between the disk and head rail surfaces, further increasing friction and decreasing disk lifetime.
A standard of 10,000 stop/start cycles before disk wear is a standard established by the American National Standards Institute (ANSI) for acceptable disk performance and is the generally accepted minimum standard in the industry. In practice, moreover, this standard must be surpassed by comfortable margins for disks to be acceptable by users. Since for thin film disks obtaining high density recording of 15000 bpi and above entails flying the head as close to the magnetic layer as possible, the protective overcoat on top of the magnetic layer must be thinner than 800 .ANG. and is typically around 300 .ANG.. This extreme thinness of the outer protective layer means that the protective layer is particularly vulnerable to wear. Hence, any technique that significantly increases the number of start/stop cycles before disk wear occurs would be valuable to the manufacturers of thin film disks.